In general, a fuel injection control system of an internal combustion engine includes a fuel injector of an electromagnetic drive type and calculates a required injection amount according to an operating state of the internal combustion engine and opens the fuel injector by an injection pulse of a pulse width corresponding to the required injection amount.
However, in the fuel injector of the internal combustion engine of a direct injection type which injects fuel of high pressure into a cylinder, linearity of a change characteristic of an actual injection amount with respect to an injection pulse width tends to become worse in a partial lift range (range in which an injection pulse width is short and hence in which a lift amount of a valve body is brought into a partial lift state in which the lift amount does not reach a full lift position). In this partial lift range, variations in the lift amount of the valve body (for example, a needle valve, an armature) tend to become large and variations in an injection amount tend to become large. When the variations in the injection amount become large, an exhaust emission and drivability is likely to become worse.
As to a technique relating to correction of the variations in the injection amount of the fuel injector, there is proposed a technique that detects a behavior of an armature of a solenoid on the basis of a behavior of a drive voltage (for example, a minus terminal voltage) of the fuel injector, for example, as disclosed in a patent literature 1, a technique that compares a drive voltage of a solenoid with a reference voltage obtained by filtering the drive voltage by a low-pass filter to thereby detect a position of an armature of the solenoid on the basis of an intersection of both of them. In this technique, variations in the lift amount of the valve body of the fuel injector are estimated on the basis of the detected behavior of the armature and variations in the injection amount are corrected.
However, it is general that the behavior of the drive voltage of the fuel injector is affected by an individual difference of a magnetic circuit of the fuel injector and hence that variations are caused in the behavior of the drive voltage between the individual magnetic circuits. In order to improve detection accuracy of the behavior of the armature of the solenoid (that is, a behavior of the valve body of the fuel injector) on the basis of the behavior of the drive voltage, it is desirable that a degree of variation in the behavior of the drive voltage, which is affected by the individual difference of the magnetic circuit of the fuel injector, can be detected with high accuracy.